Temperature control for an indirectly heated cathode for a high power electron beam gun

ABSTRACT

A method and apparatus for regulating and determining the temperature of a heated cathode in an electron beam system to regulate the magnitude of the beam current both during operation and prior to turn on. This is accomplished by measuring the thermionic emission of a well defined area of the cathode, regardless of whether the electron beam is being drawn from the cathode, and utilizing this measured signal to control the temperature of the heated cathode.

United States Patent Schumacher 1 May 20, 1975 [54] TEMPERATURE CONTROLFOR AN 2,677,787 5/1954 Litton 315/107 x 2,945,100 7/1960 Burk 315/107 xINDIRECT HEATED CATHODE FOR A 2,954,470 9/1960 Brashear 315/107 x HIGHPOWER ELECTRON BEAM GUN Berthold W. Schumacher, Pittsburgh, Pa.

Westinghouse Electric Corporation, Pittsburgh, Pa,

Filed: Apr. 30, 1973 Appl. No.: 356,018

Inventor:

Assignee:

References Cited UNITED STATES PATENTS 11/1950 Lawrence et al, 315/107 XWIN 1 CAL PUMP DIFFUSION PUMP MECHANlCAL PUMP ECHANICAL PUMP PmT ECTWEGAS Primary Examiner-Nathan Kaufman Attorney, Agent, or Firm-W. G,Sutclifi [57] ABSTRACT A method and apparatus for regulating anddetermining the temperature of a heated cathode in an electron beamsystem to regulate the magnitude of the beam current both duringoperation and prior to turn on. This is accomplished by measuring thethermionic emission of a well defined area of the cathode, regardless ofwhether the electron beam is being drawn from the cathode, and utilizingthis measured signal to control the temperature of the heated cathode.

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TEMPERATURE CONTROL FOR AN INDIRECTLY HEATED CATHODE FOR A HIGH POWERELECTRON BEAM GUN BACKGROUND OF THE INVENTION In high power electronbeam systems, such as an electron beam welder, it is often necessary tocontrol the magnitude of the beam current very closely and also in someapplications to predict the current that will appear at the firstinstance that the beam is gated on by application of a high acceleratingvoltage. The dominating parameter in most high energy electron beamsystems which determines the current in the electron beam is thetemperature of the cathode. It is found that determining the temperaturefrom the emission current of such a cathode is a very accurate way tomeasure the current. It is know in the art to stabilize the current inthe electron beam by means of a feedback loop that controls either thecathode heating power or a grid control. It is however important to beable to measure the cathode temperature prior to turning the electronbeam on to insure that a proper amount of electron beam current isapplied in a particular application. An excessive amount of current atthe time of turn on of the electron beam can result for instance increating a burn rather than a good weld.

SUMMARY OF THE INVENTION In accordance with the present invention, anauxiliary electrode is associated with the cathode to derive electronemissions from the cathode from an area isolated from the electron beamsource area to obtain a sensing signal representative of the temperatureof the cathode. The signal thus derived by means of the auxiliaryelectrode may be utilized to control the heating power source connectedto the cathode. In addition, the operating potential of the auxiliaryelectrode is such as to provide no substantial loading on the cathode,and the auxiliary electrode is shielded to provide a signalrepresentative of a given area.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of theinvention, reference may be had to the preferred embodiment, exemplaryof the invention, shown in the accompanying drawings, in which:

FIG. 1 is a schematic showing of an electron beam welding apparatusincorporating the teachings of this invention;

FIG. 2 is an enlarged view ofa portion of the electron gun illustratedin FIG. 1;

FIG. 3 is a schematic showing of a circuit that may be associated withthe assembly shown in FIG. 2; and

FIG. 4 illustrates a possible modification of the cathode structureillustrated in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring in detail to FIG. 1,an electron beam apparatus for use in the atmosphere is illustrated. Theapparatus basically consists of three sections l0, l2 and I4. The topsection or chamber is a high pressure section which is filled with asuitable insulating gas, such as F5 and wherein the high voltagesupplies are located. The middle section 12 contains the electron gunproper and is the section of the apparatus where the lowest vacuum isfound. A cathode assembly 16 and anode 17 are mounted in the middlesection 12. The next chamber or lower chamber 14 is where the electronbeam generated by the cathode assembly 16 is directed and passes throughsucceeding pumping stages and finally to a bottom orifice 18 where itgoes on into the full atmosphere and bombards a workpiece 20.

Several pumping stages and several pumping orifices are illustrated inFIG. 1. The pressure increases stage by stage from the cathode assembly16 on down to the workpiece 20. The cathode environment is at a pressureof about 10 Torr. The succeeding stage is at a fraction of a Torr, andthe final stage is about 200 Torr. A magnetic lens 24 may be utilizedfor focusing the electron beam.

The electron beam generated at the cathode assembly 16 is accelerated byan anode member 17. The anode member 17 is at substantially groundpotential. The cathode assembly 16 is at a negative potential of about150,000 volts, while the succeeding stages of the system are allactually at ground potential. The electron beam proceeds through anaperture in the anode member 17 and is focused by the magnetic lens 24and passes on through several orifices separating differential pressureareas which are maintained by the various pumps located progressivelyalong the tubular housing 26. The housing 26 may be of a suitable metalsuch as non-magnetic stainless steel.

FIG. 2 illustrates the cathode assembly 16. The assembly 16 consists ofabase member 30 from which extends a tungsten rod, bolt cathode 32. Theelectron beam is drawn from the end face portion 34 of the bolt cathode32. An auxiliary cathode 36, here a tungsten filament, surrounds thebolt cathode 32, and generates electrons by which the bolt cathode isbombarded and heated. The auxiliary cathode 36 is maintained at anegative potential relative to bolt cathode 32. A radiation shield 38 isprovided about the auxiliary cathode 36. A grid member or focusingelectrode 39 may be provided surrounding the emission region 34.Positioned between the secondary cathode 36 and the base 30 is anauxiliary electrode 40, which typically serves as an anode, which may bea circular wire of a suitable material, such as tungsten, surroundingthe bolt cathode 32. An isolated lead-in member 42 is connected to theauxiliary anode 40 and passes through the base member 30. a shieldmember 44, effective against elective fields, for instance from thefilament cathode 36, is provided about the auxiliary anode 40 asillustrated, and consists of an annular member with an upper and lowerinturned flange member 46. The shield member 44 may be connected to thesame potential as the bolt cathode 32.

FIG. 3 illustrates a suitable circuit associated with the cathodeassembly 16. The auxiliary anode 40 is connected through a variablepotential source 46 and a sensing member 48 to the cathode 32. Thesensing member 48 may be a current measuring device of the appropriatesensitivity and is scaled to indicate the cathode temperature as afunction of the measured current. A signal amplifier 50 may be connectedacross the sensing element 48 to derive and amplify the signalsgenerated therein. A filament power supply 52 is connected across theterminals of the secondary cathode 36 to provide the necessaryexcitation and heating of the secondary cathode to generate electronsfor bombardment of the bolt cathode 32. A voltage supply 54 is connectedbetween the filament power supply and the bolt cathode 32 so as toprovide a positive potential of about 100 to 600 volts between thesecondary cathode 36 and the bolt cathode 32, so that the bolt cathodeis at a more positive potential than the secondary cathode. A controlsignal derived from the signal amplifier 50 may be utilized to modifyeither the potential of the source 54 to thereby reduce the energy ofthe electrode bombarding the cathode, or may control the potentialsupply to the filament supply 52.

In the operation of the device. the temperature of the stem of the boltmember 32 is determined by measuring the thermionic emission from a welldefined area of the stem. This area is of course beneath the auxiliaryanode 40 and limited by shield 44. This thermionic emission will beobtained regardless of whether of not the main electron beam generatedfrom the face 34 of the cathode is being drawn due to a highaccelerating potential applied. The auxiliary electrode 40 may be placedat any convenient location opposite a section of the stem. The exactlocation is immaterial since a defined and ascertainable relationshipexists between the temperature at the face 34 of the cathode and at anysection along the stem. The auxiliary anode 40 may be of a suitablematerial such as tungsten. While the auxiliary anode 40 is shown here asa ring member, it can also be a pin-like member. The anode should beshielded, by shield member 44 of a suitable material such as tungsten ortantalum, against radiations and electrons that might arrive from thesecondary cathode 36. A positive potential may be applied to theauxiliary electrode 40 with re spect to the bolt cathode 32 such thatall electrons are collected within the enclosed section of the boltstem. This current is an indication of the bolt temperature.

Instead of placing a positive potential on the auxiliary electrode 40,it is also possible to utilize this electrode 40 at zero or a slightlynegative potential. A negative potential of about 1 volt, or a variablepotential from about volts to 0 volts may be placed on the auxiliaryelectrode 40. A residual current will flow between bolt cathode 32 andelectrode 40 which is also a measure of the temperature. This lattermethod has the advantage that no great amount of power is dissipated tothe auxiliary electrode 40 from the cathode 32. The associated circuitryresponsive to the temperature signal indication may regulate via thefeedback loop either the filament supply or the accelerating voltagebetween the secondary cathode 36 and the bolt cathode 32.

In high power electron beam devices of the type described herein, thebolt cathode 36 typically is maintained at a high negative potential,from 1 W to several hundred kilovolts, The shield 44 is maintained atthe same high negative potential as the cathode 36, while the auxiliaryelectrode 40 is maintained at for example about a 100 V more positivepotential, The current flow from the cathode 36 to the auxiliaryelectrode 40 is typically of the order of a few milliamps to severalhundred milliamps. The secondary current thus involves relatively lowpower as compared to the much higher power of the main electron beam,which will have a comparable current magnitude but over a much greaterpotential difference.

The circuit in which the secondary current flows will be at the samehigh negative potential relative to ground as the bolt cathode, and thiscomplicates the current measurement and conversion to a readout which isa function of cathode temperature. In order to transmit this reading toground the sensing member 48 can be a device in which the amplifiedcurrent is passed through a filament, and the filament temperature andcurrent flow therethrough are read with an optical pyrometer to avoidthe high voltage problem. Suitable high voltage isolating step-downsystems can also be used as part of the sensing member 48. Insofar asthe signal amplifier 50 is also at a high negative potential it is alsopossible to avoid any connection between sensing member 48 and groundand simply to pass-on the signal from 48 to amplifier 50, but to adjustthe reference value which is provided for amplifier 50 through anisolation link from ground, which can be an isolated potentiometer driveelectrically connected to the amplifier 50.

While it is preferred for sensitivity purposes, that some potentialdifference be maintained between the cathode and the auxiliaryelectrode, it is within the scope of this invention to measure thethermionic emission from the heated cathode without need for maintaininga potential difference therebetween.

FIG. 4 illustrates a possible modification of the cath ode assemblywherein a strap cathode member of a suitable material such as tungstenand indicated as item is provided between two electrode supports 62 and64. The strap 60 is provided with a form dimple 66 with the emissionsurface 68 thereon for generating the electron beam. A grid member 69may be provided proximate the emission surface 68. A pin-type auxiliaryelectrode 70 may be positioned within this dimple as illustrated, or theauxiliary anode 70 may be disposed anywhere along the filament a'nd ashielding member provided. In this cathode, of course, the cathode isheated directly by passing current through the cathode or filament strap60.

Again, the current flow between the strap cathode 60 and moreparticularly, the dimple portion 66 and the auxiliary electrode 70 issensed and used to develop a feedback signal for controlling the heatingcurrent through strap cathode 60.

The apparatus and method of the present invention thus permit anaccurate temperature determination of the electron beam cathode prior touse of the main beam. This prevents for instance excessive initialcurrents which might burn through the work piece in a weldingapplication, or low current flows which would mean a poor weld.Following initiation of the main electron beam, the cathode temperaturecould continue to be monitored and controlled utilizing the presentinvention or a conventional technique may be used which senses the mainelectron beam current.

I claim as my invention:

1. ln combination with an electron gun for generating a high powerelectron beam, which electron gun includes an elongated cathode memberand cathode heater means associated therewith for electron bombardmentheating of the cathode member, with the electron beam being drawn fromone end of said cathode member, and wherein the temperature of thecathode member significantly affects the electron beam emission, theimprovement comprising a ring conductive electrode disposed about thecathode member at a location spaced from the electron beam emission endportion of the cathode member, with a secondary emit ted electroncurrent flow produced between said cathode member and said ringconductive electrode, a shielding enclosure disposed about the ringconductive electrode with the cathode member passing through ode member.

3. The combination specified in claim 1 wherein the electron gun isdisposed within a partially evacuated electron beam generating chamberof a high power electron beam system, and wherein said sensing means andcontrol signal means associated with the electron gun are operative tosense said electron emission capability of the cathode of said gun withor without flow of the electron beam current.

1. In combination with an electron gun for generating a high powerelectron beam, which electron gun includes an elongated cathode memberand cathode heater means associated therewith for electron bombardmentheating of the cathode member, with the electron beam being drawn fromone end of said cathode member, and wherein the temperature of thecathode member significantly affects the electron beam emission, theimprovement comprising a ring conductive electrode disposed about thecathode member at a location spaced from the electron beam emission endportion of the cathode member, with a secondary emitted electron currentflow produced between said cathode member and said ring conductiveelectrode, a shielding enclosure disposed about the ring conductiveelectrode with the cathode member passing through the shieldingenclosure, and sensing means electrically connected to the ringconductive electrode for measuring the secondary emitted electroncurrent and indicating the temperature of the cathode member as afunction of said secondary emitted electron current.
 2. The combinationspecified in claim 1, wherein the sensing means generates a signal whichis a function of the cathode member temperature, and the sensing meansis connected to the cathode heating means, whereby the generated signalis fed back to the cathode heating means to control the temperature ofthe cathode member.
 3. The combination specified in claim 1 wherein theelectron gun is disposed within a partially evacuated electron beamgenerating chamber of a high power electron beam system, and whereinsaid sensing means and control signal means associated with the electrongun are operative to sense said electron emission capability of thecathode of said gun with or without flow of the electron beam current.